Zircon compatible, ion exchangeable glass with high damage resistance

ABSTRACT

An ion exchangeable glass having a high degree of resistance to damage caused by abrasion, scratching, indentation, and the like. The glass comprises alumina, B 2 O 3 , and alkali metal oxides, and contains boron cations having three-fold coordination. The glass, when ion exchanged, has a Vickers crack initiation threshold of at least 10 kilogram force (kgf).

This application is a continuation of U.S. application Ser. No.13/903,433 filed on May 28, 2013, which claims the benefit of priorityunder 35 U.S.C. §119 of U.S. Provisional Application Ser. No.61/653,489, filed on May 31, 2012, and U.S. Provisional Application Ser.No. 61/748,981, filed on Jan. 4, 2013, the contents of all of the abovebeing incorporated herein by reference in their entirety.

BACKGROUND

The disclosure relates to glasses that are capable of chemicalstrengthening by ion exchange and have intrinsic or “native” damageresistance. More particularly, the disclosure relates to such glassesthat are strengthened by ion exchange and possess resistance to damageby abrasion, scratching, indentation, and other forms of sharp contact.

The ion exchange process provides a substantial improvement of glassesthat are capable of being strengthened by this process to resist damageby sharp impact or indentation. To date, glasses containing networkmodifiers such as alkali and alkaline earth cations have been used.These cations form non-bridging oxygens (oxygens bonded to only onesilicon atom), which reduce the resistance of the ion exchanged glass todamage introduced by abrasion, scratching, or the like.

SUMMARY

The present disclosure provides an ion exchangeable glass having a highdegree of resistance to damage caused by abrasion, scratching,indentation, and the like. The glass comprises alumina, B₂O₃, and alkalimetal oxides, and contains boron cations having three-fold coordination.The glass, when ion exchanged, has a Vickers crack initiation thresholdof at least 10 kilogram force (kgf).

Accordingly, one aspect of the disclosure is to provide an ionexchangeable glass comprising at least about 50 mol % SiO₂; at leastabout 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol%)<R₂O(mol %); and B₂O₃, and wherein B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol%))≧3 mol %.

A second aspect of the disclosure is to provide a glass comprising atleast about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂Ocomprises Na₂O; Al₂O₃; and B₂O₃, wherein B₂O₃(mol %)-(R₂O(mol%)-Al₂O₃(mol %))≧3 mol %, and wherein the glass has a zircon breakdowntemperature that is equal to the temperature at which the glass has aviscosity in a range from about 25 kPoise to about 40 kPoise.

A third aspect of the disclosure is to provide an ion exchanged glasshaving a Vickers crack initiation threshold of at least about 10 kgf.The glass comprises at least about 50 mol % SiO₂; at least about 10 mol% R₂O, wherein R₂O comprises Na₂O; Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol%); and B₂O₃, wherein B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %.

A fourth aspect is to provide a glass comprising: at least about 50 mol% SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃;and at least 2.7 mol % B₂O₃ containing coordinated boron cations,wherein B₂O₃—(R₂O—Al₂O₃)≧3 mol %.

A fifth aspect is directed to a glass comprising: at least about 50 mol% SiO₂; at least about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃,wherein Al₂O₃(mol %)<R₂O(mol %); and 3-4.5 mol % B₂O₃, and whereinB₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %, and wherein the glass ision exchangeable.

A sixth aspect is directed to a glass comprising: 66-74 mol % SiO₂; atleast about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, whereinAl₂O₃(mol %)<R₂O(mol %); and B₂O₃, wherein 4.5 mol %≧B₂O₃(mol%)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %, and wherein the glass is ionexchangeable.

A seventh aspect is directed to a glass comprising 66-74 mol % SiO₂; atleast about 10 mol % R₂O, wherein R₂O comprises Na₂O; Al₂O₃, whereinAl₂O₃(mol %)<R₂O(mol %); and 3-4.5 mol % B₂O₃, wherein B₂O₃(mol%)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %, wherein the glass comprisesthree-fold coordinated boron cations, and the glass is ion exchangeable.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of nuclear magnetic resonance (NMR) spectra for samplesof the glasses described herein.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may consist of any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified. It also is understood that the various featuresdisclosed in the specification and the drawings can be used in any andall combinations.

As used herein, the terms “glass” and “glasses” includes both glassesand glass ceramics. The terms “glass article” and “glass articles” areused in their broadest sense to include any object made wholly or partlyof glass and/or glass ceramic.

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue.

Unless otherwise specified, all compositions and concentrationsdescribed herein are expressed in mole percent (mol %). It is understoodthat the compositional ranges described and claimed herein represent thebulk composition of the glass as determined by those means skilled inthe art, and are applicable to both the unstrengthened and strengthenedglasses described herein.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

Accordingly, a glass having intrinsic damage resistance is provided. Inone aspect, the glass, which may be strengthened so as to be resistantto damage via sharp impact, comprises at least about 50 mol % SiO₂; atleast about 10 mol % of at least one alkali metal oxide R₂O, wherein R₂Oincludes Na₂O and, optionally, other alkali metal oxides (e.g., Li₂O,K₂O, Ce₂O, Rb₂O); alumina (Al₂O₃), wherein the amount of Al₂O₃,expressed in mol %, is less than the total amount of the alkali metaloxides present in the glass (i.e., Al₂O₃(mol %)<R₂O(mol %)); and boronoxide (B₂O₃), wherein B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %.

In another aspect, the glass comprises at least about 50 mol % SiO₂;Al₂O₃; B₂O₃; and at least about 10 mol % R₂O, wherein R₂O comprisesNa₂O; wherein B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %, and whereinthe glass has a zircon breakdown temperature (i.e., the temperature atwhich zircon breaks down to form zirconia and silica) equal to thetemperature at which the viscosity of the glass is in a range from about25 kPoise to about 40 kPoise.

In some aspects, the glasses described herein are ion exchangeable;i.e., cations—typically monovalent alkali metal cations—which arepresent in these glasses are replaced with larger cations—typicallymonovalent alkali metal cations, although other cations such as Ag⁺ orTl⁺—having the same valence or oxidation state. The replacement ofsmaller cations with larger cations creates a surface layer that isunder compression, or compressive stress CS. This layer extends from thesurface into the interior or bulk of the glass to a depth of layer DOL.The compressive stress in the surface layers of the glass are balancedby a tensile stress, or central tension CT, in the interior or innerregion of the glass. Compressive stress and depth of layer are measuredusing those means known in the art. Such means include, but are notlimited to measurement of surface stress (FSM) using commerciallyavailable instruments such as the FSM-6000, manufactured by Luceo Co.,Ltd. (Tokyo, Japan), or the like. Methods of measuring compressivestress and depth of layer are described in ASTM 1422C-99, entitled“Standard Specification for Chemically Strengthened Flat Glass,” andASTM 1279.19779 “Standard Test Method for Non-Destructive PhotoelasticMeasurement of Edge and Surface Stresses in Annealed, Heat-Strengthened,and Fully-Tempered Flat Glass,” the contents of which are incorporatedherein by reference in their entirety. Surface stress measurements relyupon the accurate measurement of the stress optical coefficient (SOC),which is related to the stress-induced birefringence of the glass. SOCin turn is measured by those methods that are known in the art, such asfiber and four point bend method, both of which are described in ASTMstandard C770-98 (2008), entitled “Standard Test Method for Measurementof Glass Stress-Optical Coefficient,” the contents of which areincorporated herein by reference in their entirety, and a bulk cylindermethod.

In other aspects, the glasses described herein are ion exchanged andhave at least one surface having a layer that is under compression to adepth of layer and an inner region that is under a tensile stress, asdescribed hereinabove. In one aspect, the ion exchanged glass comprisesat least about 50 mol % SiO₂; at least about 10 mol % R₂O, wherein R₂Ocomprises Na₂O; Al₂O₃; and B₂O₃, and wherein B₂O₃—(R₂O—Al₂O₃)≧3 mol %.The ion exchanged glass has a Vickers crack initiation threshold of atleast about 10 kilogram force (kgf).

When present in boroaluminosilicate glasses, alkali metals act ascharge-balancing cations for tetrahedrally coordinated aluminum andboron. Species similar to RAlO₂ or RBO₂, where R is an alkali metal andaluminum are Al³⁺ and B³⁺, for example, are four-fold coordinated byoxygen. The four oxygen atoms surrounding aluminum or boron are presumedto bond to silicon atoms. Each oxygen therefore obtains one electronfrom the silicon atoms and, in effect, ¾ of an electron from thetetrahedrally coordinated Al³⁺ or B³⁺. An alkali metal donates anelectron to this configuration, thus allowing all four oxygens in thetetrahedron to obtain a full electron shell. The large alkaline earthelements Ca, Sr, and Ba are known to fill this role for aluminum. Thus,in a boroaluminosilicate glass in which R₂O(mol %)+CaO(mol%)+SrO+BaO(mol %)-Al₂O₃(mol %)-B₂O₃(mol %)=0(mol %) or, alternatively,(Al₂O₃(mol %)+B₂O₃(mol %))/(R₂O(mol %)+CaO(mol %)+SrO(mol %)+BaO(mol%))=1, there are exactly as many electrons donated from alkalis andalkaline earths as there are tetrahedrally coordinated aluminum andboron atoms in need of charge balance. When R₂O(mol %)+CaO(mol%)+SrO(mol %)+BaO(mol %)-Al₂O₃(mol %)-B₂O₃(mol %)<0(mol %), there aremore aluminum and boron atoms in need of charge balance than electronsfrom charge-balancing alkali or alkaline earth cations. Such glassesgenerally have high intrinsic or “native” damage resistance, which isconsiderably enhanced after ion exchange.

In these glasses, aluminum is nearly always four-fold coordinated byoxygen if enough charge-balancing cations are available. Low-valencecations perform better in a charge-balancing role than small, highervalence cations. This has been attributed to the extent to which thecharge-balancing cation competes with aluminum for electron density.Large, low-valence cations are ineffective in this competition and thusleave electron density largely within the AlO₄ tetrahedron. Boron ismostly four-fold coordinated in fully annealed alkali borosilicateglasses in which the amount of alkali oxide present in the glass is lessthan or equal to than the concentration Al₂O₃, as the alkali modifiersare consumed or taken up in bonds with aluminum. In fully annealedalkali borosilicate glasses in which the amount of alkali oxide presentin the glass is greater than the combined concentration of B₂O₃ andAl₂O₃, boron is mostly four-fold coordinated and the excess alkalimodifiers are taken up in non-bridging oxygen (NBO) bonds. In fullyannealed boroaluminosilicate glasses in which B₂O₃≧alkalimodifiers>Al₂O₃, however, boron assumes a mix of three and four-foldcoordination, even in charge-balanced compositions. When fast-cooledfrom above the anneal or strain point of the glass, the formation ofthree-fold coordinated boron is entropically preferred. Thus, theproportion of three-fold coordinated boron may be enhanced throughforming processes such as down-draw (e.g., slot- or fusion-draw)processes, or by heating the glass to a temperature above tits anneal orstrain point, followed by fast-cooling (e.g., at rates of at least about4° C./s). In some embodiments, the glass is fast-cooled from atemperature that is above the anneal or strain point of the glass. To areasonable approximation, charge-balancing cations first stabilizealuminum in four-fold coordination, and the remaining cations stabilizesome fraction of boron in four-fold coordination, where the fraction maybe as high as 1. The glasses described herein comprise at least 2.7 mol% and, in some embodiments, from about 2.7 mol % to about 4.5 mol %B₂O₃, in which the boron cations are predominantly three-foldcoordinated. In some embodiments, at least about 50% of the boroncations comprising the glasses described herein are three-foldcoordinated. In other embodiments, at least about 80% of the boron inthese glasses is three-fold coordinated and, in still other embodiments,at least 90% of the boron cations in these glasses are three-foldcoordinated. The proportion of three fold boron can be determined using¹¹B nuclear magnetic resonance (NMR). FIG. 1 shows NMR spectra forsamples of the glasses described herein having the composition 67.45 mol% SiO₂, 12.69 mol % Al₂O₃, 3.67 mol % B₂O₃, 13.67 mol % Na₂O, 2.36 mol %MgO, and 0.09 mol % SnO₂. The amount of four-fold coordinated boron (N₄in FIG. 1) ranges from 8.9% to 9.5%. Thus, about 90% of the boronpresent in each of the samples shown in FIG. 1 is three-foldcoordinated.

When R₂O(mol %)+MO(mol %)<Al₂O₃(mol %), where MO represents divalentmetal oxides (e.g., MgO, ZnO, CaO, BaO, SrO, etc.), there are not enoughmonovalent or divalent cations to stabilize aluminum in four-foldcoordination, and boron is therefore almost entirely in three-foldcoordination in these glasses. In some embodiments, the glassesdescribed herein comprise about 3 mol % to about 4.5 mol % of B₂O₃ whichcontains boron cations that are surrounded by three anions; i.e., theboron cations are three-fold coordinated and, in some embodiments,B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≦4.5 mol %. Three-fold coordinatedboron is desirable because it densifies more readily when compressed,thus providing intrinsic or native damage resistance to the glass.Glasses with compositions in which R₂O(mol %)+MO(mol %)-Al₂O₃(mol%)<0(mol %), however, generally have very high melting temperaturesunless they also have very high B₂O₃ contents, e.g., greater than about10 mol %. Such high amounts of B₂O₃ have a deleterious effect on boththe surface compressive stress and rate of ion exchange, as evidenced bylow depths of layer obtained in an ion exchange of fixed duration/timeperiod. Since high CS (e.g., greater than about 650 MPa) and significantdepth of layer (DOL) (e.g., greater than 30 μm) are also required toobtain good damage resistance, the additional benefits of high intrinsicdamage resistance may be offset by poor ion exchange characteristics. Inorder to achieve optimal balance of attributes after ion exchange, it istherefore desirable to keep boron concentrations sufficiently low.Accordingly, in some embodiments, the glasses described herein compriseless than 10 mol % B₂O₃. Such glasses are very difficult to melt to alevel of quality (e.g., defect concentration) acceptable for massmanufacturing. Depending on the application for the glass article, amarginally acceptable defect level in a continuous process is about 1inclusion (e.g., gas bubbles, particles, or “seeds”) that are greaterthan about 50 μm in size (e.g., diameter, for approximately sphericalparticles, or major axis for elongated particles) per pound (lb) ofglass.

In some embodiments, the glasses described herein comprise at leastabout 50 mol % SiO₂, from about 9 mol % to about 22 mol % Al₂O₃; fromabout 3 mol % to about 10 mol % B₂O₃; from about 10 mol % to about 20mol % Na₂O; from 0 mol % to about 5 mol % K₂O; at least about 0.1 mol %MgO and/or ZnO, wherein 0 mol %≦MgO(mol %)+ZnO(mol %)≦6 mol %; and,optionally, at least one of CaO, BaO, and SrO, wherein 0 mol %≦CaO(mol%)+SrO(mol %)+BaO(mol %)≦2 mol %. In some embodiments, the glassesdescribed herein comprise from about 66 mol % to about 74 mol % SiO₂.

In some embodiments, the glasses described herein comprise at leastabout 0.1 mol % of at least one of MgO and ZnO. In certain embodiments,the glass may comprise up to about 6 mol % MgO and/or ZnO. Magnesium andzinc behave differently from the other divalent cations. While magnesiumand zinc may charge-balance aluminum to some extent, they appear to havelittle or no role in stabilizing boron in four-fold coordination in theglasses described herein. Thus, replacing an alkali oxide with magnesiumoxide in a boroaluminosilicate glass forces boron atoms out of four-foldcoordination and into three-fold coordination, resulting in higherindentation crack initiation threshold load in ion-exchanged glass.

In addition to the oxides listed above, the glasses described herein mayfurther comprise chemical fining agents including, but not limited to,halogens or halides (compounds containing F, Cl, Br and I) and at leastone of the oxides As₂O₃, Sb₂O₃, CeO₂, and SnO₂. These fining agents,when present, are generally batched into the raw material at a level ofabout 0.5 mol % or less, and have minor impact on both the rate of ionexchange and the compressive stress that are ultimately obtained. Inaddition, other oxides may be added at low concentrations with little orno impact on the ion exchange characteristics of these glasses. Examplesof such oxides include ZrO₂, which is a common contaminant introduced byzirconia refractories in melters; TiO₂, which is a common trampcomponent of natural silica sources; Fe₂O₃, which is an ubiquitous trampoxide in all but the purest chemical reagents; and other transitionmetal oxides that may be used to introduce color. These transition metaloxides include V₂O₅, Cr₂O₃, Co₃O₄, Fe₂O₃, MnO₂, NiO, CuO, Cu₂O, and thelike. Concentrations of these oxides, when present in the glass, arekept at levels of less than or equal to about 2 mol %. Compared tocalcium, larger alkaline earths such as Sr and Ba lead to lowerdiffusivity and thus significantly lower depth of the compressive layer.Because Sr and Ba are costly reagents compared to most of the otheroxides in the glasses described herein and inhibit or retard ionexchange. The concentrations of SrO and BaO, when present in the glass,are therefore kept at levels of no more than about 0.5 mol %. Rubidiumand cesium oxides are too large to ion exchange at an appreciable rate,are costly, and contribute to high liquidus temperatures at elevatedconcentrations. The concentrations of these oxides are therefore keptbelow 0.5 mol %. Lithium oxide is to be generally avoided, as itcontributes to “poisoning” of potassium nitrate salt ion exchange baths,resulting in lower compressive stress than if the same glass were ionexchanged in a lithium-free salt bath. In addition, compared to asodium-for-potassium exchange, the presence of lithium leads tosignificantly reduced diffusivity when exchanging for potassium. Whenpresent in the glass, the lithium oxide concentration should thereforebe kept below about 0.5 mol %, and, in some embodiments, below about 0.1mol %. In some embodiments, the glasses described herein aresubstantially free of lithium. In some embodiments, the glassesdescribed herein are substantially free of phosphorus.

As used herein, the term “zircon breakdown temperature” or“T^(breakdown)” refers to the temperature at which zircon—which iscommonly used as a refractory material in glass processing andmanufacture—breaks down to form zirconia and silica. In isoviscousprocesses such as fusion, the highest temperature experienced by theglass corresponds to a particular viscosity of the glass. For example,“T^(35 kP)” refers to the temperature at which the glass has a viscosityof 35 kilopoise (kP). The difference between the breakdown temperatureand the temperature corresponding to 35,000 poise viscosity is definedas the breakdown margin T^(margin), where:T^(margin)=T^(breakdown)−T^(35 kp). When the breakdown margin T^(margin)is negative, zircon will breakdown to form zirconia defects at somelocation on the fusion isopipe. When T^(margin) is zero, it is stillpossible that temperature excursions could cause zircon breakdown tooccur. It is therefore desirable not only to make the breakdown marginpositive, but to maximize T^(margin) as much as possible while beingconsistent with all the other attributes that must be maintained in thefinal glass product. In some embodiments, the glasses described hereinhave a zircon breakdown temperature that is equal to the temperature atwhich the viscosity of the glass is in a range from about 25 kPoise toabout 40 kPoise, in some embodiments, in a range from about 30 kPoise toabout 40 kPoise, and, in a particular embodiment, from 30 kPoise toabout 35 kPoise. Zircon breakdown temperatures T^(breakdown) measuredfor various samples of the glasses described herein are listed in Table3.

As previously mentioned, zircon will breakdown to form zirconia andsilica at some location on processing hardware—such as, for example, afusion isopipe—when the breakdown margin T^(margin) is negative. Thisoften results in the inclusion of defects, bubbles, zirconia particles,or the like in the formed glass. The presence of such inclusions in theglass is typically detected by those means know in the art such as, forexample, optical microscopy. The glasses described herein have positivezircon breakdown margins and, in some embodiments, have less than 0.1inclusions per pound of glass, wherein the inclusions are at least about50 μm in size (i.e., diameter, for approximately spherical particles, ormajor axis for elongated particles). In some embodiments, the glassesdescribed herein contain less than 0.01 inclusions per pound of glass,wherein the inclusions are at least about 50 μm in size.

In those embodiments in which the glass is strengthened by ion exchange,the glass may, in some embodiments, be ion exchanged to create a surfacelayer that is under a compressive stress of at least about 600megapascals (MPa) and, in other embodiments, at least about 800 MPa. Thecompressive surface layer has a depth of layer of at least about 30microns (μm). The ion exchanged glasses described herein also possess adegree of native or intrinsic damage resistance (IDR), which may becharacterized by a Vickers crack initiation threshold of greater than orequal to 10 kilogram force (kgf). In some embodiments, the ion exchangedglass has a Vickers crack initiation threshold in a range from about 20kgf to about 30 kgf. In other embodiments, the ion exchanged glass has aVickers crack initiation threshold in a range from about 30 kgf to about35 kgf. The Vickers crack initiation threshold measurements describedherein are performed by applying and then removing an indentation loadto the glass surface at a rate of 0.2 mm/min. The maximum indentationload is held for 10 seconds. The crack initiation threshold is definedat the indentation load at which 50% of 10 indents exhibit any number ofradial/median cracks emanating from the corners of the indentimpression. The maximum load is increased until the threshold is met fora given glass composition. All indentation measurements are performed atroom temperature in 50% relative humidity.

When ion exchanged, the glasses described herein have a compressivestress that is sufficiently high to provide outstanding damageresistance against impact of various kinds. The glasses described hereinmay be ion exchanged at a rate that facilitates large scalemanufacturing. While it is advantageous to provide high compressivestress by annealing the glass prior to ion exchange, a high compressivestress may also be obtained by instead cooling the glass rapidly from ahigh temperature (e.g., above the strain point of the glass). Such rapidcooling may occur, for example, in down drawn processes such as fusionor slot draw processes. The combination of compressive stress, depth oflayer, and intrinsic damage resistance of the ion exchanged glassesdescribed herein provides excellent resistance to visible orstrength-limiting damage introduced via sharp contact and scratching.

Physical properties of an exemplary sample of the glasses describedherein, labeled Glass 3 and having the composition 67.45 mol % SiO₂,12.69 mol % Al₂O₃, 3.67 mol % B₂O₃, 13.67 mol % Na₂O, 0.02 mol % K₂O,2.36 mol % MgO, 0.03 mol % CaO, and 0.09 mol % SnO₂, are compared tothose of glasses described in U.S. patent application Ser. No.13/533,298, filed on Jul. 1, 2011, by Matthew J. Dejneka et al.,entitled “Ion Exchangeable Glass With High Compressive Stress,” whichclaims priority from U.S. Provisional Patent Application No. 61/503,734,filed on Jul. 1, 2011, by Matthew J. Dejneka et al., and having the sametitle; and U.S. Provisional Patent Application No. 61/653,485, filed onMay 31, 2012, by Matthew J. Dejneka et al., entitled “Ion ExchangeableGlass With High Damage Resistance;” labeled Glasses 1 and 2,respectively, in Table 1. Glass 1, having the composition 68.84 mol %SiO₂, 10.63 mol % Al₂O₃, 0 mol % B₂O₃, 14.86 mol % Na₂O, 0.02 mol % K₂O,5.43 mol % MgO, 0.04 mol % CaO, and 0.17 mol % SnO₂, is compatible withzircon, but has a low indentation threshold, whereas Glass 2, having thecomposition 64.65 mol % SiO₂, 13.93 mol % Al₂O₃, 5.11 mol % B₂O₃, 13.75mol % Na₂O, 0 mol % K₂O, 2.38 mol % MgO, 0.14 mol % CaO, and 0.08 mol %SnO₂, has a high indentation threshold but is incompatible with zircon.Glass 3 possesses a combination of high indentation threshold andcompatibility with zircon, as described hereinabove.

TABLE 1 Physical properties of glasses. Glass 1 2 3 Anneal Point (° C.)650 629 644 Strain Point (° C.) 601 576 589 Softening Point (° C.) 891.7900 922.4 Density (g/cm³) 2.432 2.39 2.403 Poisson's Ratio 0.205 0.2160.213 Shear Modulus (Mpsi) 4.287 4.051 4.142 Young's Modulus (Mpsi)10.336 9.851 10.046 Liquidus Temperature (° C.) 1020 1000 1005 LiquidusViscosity (kPoise) 1000 1850 2210 Primary Devit Phase ForsteriteNepheline Forsterite Zircon Breakdown 1200 1183 1230 Temperature (° C.)Zircon Breakdown 30.4 71.4 33.4 Viscosity (kPoise) 200 Poise Temperature(° C.) 1665 1679 1757 35 kPoise Temperature (° C.): 1190 1226 1227Refractive Index 1.50030 1.49844 1.49836 SOC (nm/MPa/cm) 29.64 32.7831.94 Vickers Indentation 7 >30 20-30 Threshold (kgf)

Additional non-limiting examples of the compositions of the glassesdescribed herein and their properties are listed in Tables 2 and 3,respectively. The examples listed satisfy the requirement of havingintrinsic damage resistance, as characterized by high indentationthreshold, and compatibility with zircon, as characterized by low zirconbreakdown viscosity.

TABLE 2 Examples of compositions of the glasses described herein.Analyzed Example (mol %) 1 2 3 4 5 6 7 SiO₂ 67.26 67.47 67.37 67.4367.22 67.12 67.29 Al₂O₃ 12.05 12.08 12.07 12.03 12.03 12.03 12.05 B₂O₃2.58 2.56 2.54 2.61 2.61 2.64 2.64 Na₂O 14.14 13.08 14.10 13.10 14.2013.33 13.20 K₂O 0.01 0.96 0.01 0.96 0.03 0.94 0.96 MgO 3.80 3.69 3.343.27 3.34 3.36 2.82 CaO 0.05 0.04 0.48 0.49 0.06 0.05 0.48 ZnO 0.00 0.000.00 0.00 0.41 0.42 0.45 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂0.01 0.01 0.01 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.010.01 Total 100 100 100 100 100 100 100 (mol %) Analyzed Example (mol %)8 9 10 11 12 13 14 SiO₂ 67.25 66.32 66.32 66.22 66.26 67.28 67.29 Al₂O₃12.04 12.73 12.76 12.72 12.74 12.04 12.03 B₂O₃ 2.63 3.53 3.64 3.62 3.633.41 3.44 Na₂O 13.30 13.93 12.89 13.07 13.06 13.87 12.93 K₂O 0.96 0.030.95 0.96 0.97 0.01 0.94 Li₂O MgO 2.76 3.31 2.84 2.85 2.32 2.79 2.77 CaO0.05 0.05 0.48 0.05 0.47 0.49 0.49 ZnO 0.89 0.00 0.00 0.40 0.45 0.000.00 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂ 0.01 0.01 0.02 0.010.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 P₂O₅ 0.00 0.000.00 0.00 0.00 0.00 0.00 Total 100 100 100 100 100 100 100 (mol %)Analyzed Example (mol %) 15 16 17 18 19 20 21 SiO₂ 67.18 66.27 66.3366.16 67.23 67.61 66.82 Al₂O₃ 12.00 12.74 12.73 12.73 12.72 12.24 12.59B₂O₃ 3.39 3.54 3.53 3.58 3.63 3.64 3.51 Na₂O 14.10 14.11 14.12 14.1913.91 13.96 14.47 K₂O 0.04 0.01 0.01 0.01 0.01 0.04 0.01 MgO 1.82 2.271.79 1.84 2.34 2.35 2.45 CaO 0.49 0.05 0.04 0.48 0.05 0.06 0.05 ZnO 0.880.90 1.33 0.91 0.00 0.00 0.00 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09ZrO₂ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.010.01 0.01 Total 100 100 100 100 100 100 100 (mol %) Analyzed Example(mol %) 22 23 24 25 26 27 28 SiO₂ 66.59 67.05 66.38 66.98 67.05 67.0967.23 Al₂O₃ 12.41 12.16 12.71 12.69 12.56 12.67 12.67 B₂O₃ 3.42 2.913.56 3.98 3.99 3.62 4.10 Na₂O 13.40 13.34 14.19 13.91 13.55 14.16 13.97K₂O 0.66 0.85 0.01 0.01 0.01 0.01 0.03 MgO 3.01 2.88 1.79 2.21 2.05 2.241.83 CaO 0.12 0.06 0.04 0.03 0.03 0.03 0.06 ZnO 0.28 0.64 1.19 0.06 0.650.06 0.00 SnO₂ 0.09 0.09 0.09 0.09 0.09 0.09 0.09 ZrO₂ 0.01 0.01 0.030.01 0.01 0.01 0.01 Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Total 100100 100 100 100 100 100 (mol %) Analyzed Example (mol %) 29 30 31 32 3334 35 SiO₂ 67.31 67.32 66.96 67.43 67.09 67.45 67.11 Al₂O₃ 12.54 12.6512.63 12.56 12.66 12.46 12.57 B₂O₃ 4.25 3.76 3.96 3.93 4.15 4.07 4.12Na₂O 13.62 13.76 13.84 13.54 13.64 13.50 13.64 K₂O 0.01 0.01 0.01 0.010.01 0.01 0.01 MgO 2.11 2.37 2.47 2.41 2.33 2.38 2.42 CaO 0.04 0.04 0.040.03 0.04 0.03 0.04 ZnO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 SnO₂ 0.090.07 0.08 0.07 0.07 0.08 0.08 ZrO₂ 0.01 0.01 0.01 0.01 0.01 0.01 0.01Fe₂O₃ 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Total 100 100 100 100 100 100100 (mol %)

TABLE 3 Properties of glasses listed in Table 2. Example 1 2 3 4 5 6Anneal Point (° C.): 632 632 632 624 633 629 Strain Point (° C.): 579580 581 573 581 577 Softening Point (° C.): 890.3 895.7 888.6 884 891.5888.4 Density (g/cm³): 2.418 2.417 2.421 2.422 2.424 2.425 CTE (×10⁻⁷/°C.): 77.1 79.3 77.3 79.3 77.3 80.3 Poisson's Ratio: 0.216 0.202 0.2110.21 0.202 0.214 Shear Modulus (Mpsi): 4.216 4.232 4.234 4.241 4.2284.221 Young's Modulus 10.25 10.174 10.252 10.265 10.16 10.247 (Mpsi):72-hr Pt Liquidus Temp 1060 1070 1010 1000 1030 1040 (° C.): LiquidusViscosity 247 345 739 1458 810 810 (kPoise): Primary Devit Phase:Unknown Unknown Unknown Unknown Unknown Unknown 200-Poise Temp (° C.):1744.5 1715.2 1733.2 1724.3 1727.5 1736.6 35-kPoise Temp (° C.): 1215.21210.8 1196.8 1199.6 1206.5 1213.5 Fictivation Temp (° C.): 721.8 715.8714.3 704.5 717.2 717.1 Refractive Index: 1.50000 1.49992 1.500681.50104 1.50033 1.50051 SOC (nm/MPa/cm): 30.88 30.75 30.86 30.74 31.2231.01 CS at 50 μm 989.6 954.8 979.6 938.6 968.8 941.4 DOL/410° C./1 mm/Annealed/Refined (MPa): Time to 50 μm DOL at 18.5 13.8 20.4 15 18.3 13.1410° C./Annealed/Refined (h): Predicted Indentation 15 16 14 14 16 15Threshold at 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1210 12041207 1199 1206 1193 Temperature (° C.): Zircon Breakdown 38 39 30 35 3648 Viscosity (kPoise): Example 7 8 9 10 11 12 Anneal Point (° C.): 625629 629 622 623 620 Strain Point (° C.): 573 576 577 570 571 568Softening Point (° C.): 883.9 889.8 890.6 885.6 889.8 883.1 Density(g/cm³): 2.428 2.422 2.411 2.414 2.418 2.421 CTE (×10⁻⁷/° C.): 79.3 79.976.5 78.8 78.8 78.4 Poisson's Ratio: 0.221 0.209 0.213 0.209 0.212 0.215Shear Modulus (Mpsi): 4.241 4.231 4.164 4.197 4.189 4.193 Young'sModulus 10.36 10.228 10.106 10.148 10.153 10.192 (Mpsi): 72-hr PtLiquidus Temp 970 995 1065 1025 1040 960 (° C.): Liquidus Viscosity 27991401 287 939 805 4180 (kPoise): Primary Devit Phase: Albite UnknownUnknown Unknown Unknown Unknown 200-Poise Temp (° C.): 1721.0 1729.61716.9 1718.2 1722.0 1714.0 35-kPoise Temp (° C.): 1205.1 1205.4 1212.11209.7 1213.1 1204.1 Fictivation Temp (° C.): 1.50124 1.50102 1.499671.50039 1.50016 1.50090 Refractive Index: 30.67 31.18 31.5 31.13 31.5431.53 SOC (nm/MPa/cm): 957.4 945.5 968.3 942.5 927 927.6 CS at 50 μm14.4 13.6 18.3 15.3 13.7 15.7 DOL/410° C./1 mm/ Annealed/Refined (MPa):Time to 50 μm DOL at 11 15 20 18 18 17 410° C./Annealed/Refined (h):Zircon Breakdown 1192 1192 1202 1190 1187 1182 Temperature (° C.):Zircon Breakdown 43 43 41 48 52 49 Viscosity (kPoise): Example 13 14 1516 17 18 Anneal Point (° C.): 619 617 617 627 626 622 Strain Point (°C.): 568 567 567 574 573 571 Softening Point (° C.): 877.5 876.6 871.5890.5 889.1 881.2 Density (g/cm³): 2.415 2.415 2.429 2.425 2.432 2.43CTE (×10⁻⁷/° C.): 76.3 78.8 76.1 76.1 76.1 76.2 Poisson's Ratio: 0.2230.203 0.216 0.216 0.205 Shear Modulus (Mpsi): 4.221 4.226 4.158 4.1454.197 Young's Modulus 10.324 10.171 10.117 10.083 10.119 (Mpsi): 72-hrPt Liquidus Temp 960 950 970 970 950 965 (° C.): Liquidus Viscosity 29752742 3595 2933 3783 2470 (kPoise): Primary Devit Phase: Albite AlbiteAlbite Unknown Albite Albite 200-Poise Temp (° C.): 1692.6 1658.8 1815.21714.7 1725.1 1717.9 35-kPoise Temp (° C.): 1179.0 1165.6 1226.2 1208.51207.9 1199.5 Refractive Index: 31.07 31.11 32.52 32.02 32.19 31.79 SOC(nm/MPa/cm): 933.5 908.4 969.6 976.2 963.9 CS at 50 μm 21.8 16.4 19.719.5 21.3 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOL at13 16 20 21 19 410° C./Annealed/Refined (h): Zircon Breakdown 1218 12081205 1193 1191 1184 Temperature (° C.): Zircon Breakdown 19 18 47 45 4644 Viscosity (kPoise): Example 19 20 21 22 23 24 Anneal Point (° C.):629 621 621 629 629 626 Strain Point (° C.): 576 569 570 577 576 574Softening Point (° C.): 898.9 884.3 877.3 893.2 896.2 890.2 Density(g/cm³): 2.404 2.406 2.412 2.42 2.426 2.429 CTE (×10⁻⁷/° C.): 76.4 76.878.1 78.2 78.6 76.1 Poisson's Ratio: 0.208 0.209 0.215 0.212 0.222 0.219Shear Modulus (Mpsi): 4.141 4.161 4.152 4.222 4.222 4.158 Young'sModulus 10 10.058 10.09 10.232 10.315 10.138 (Mpsi): 72-hr Pt LiquidusTemp 980 965 960 1030 1020 955 (° C.): Liquidus Viscosity 980 950 9601020 1020 945 (kPoise): Primary Devit Phase: 3592 3855 3649 904 13585875 200-Poise Temp (° C.): Unknown Albite Albite Unknown Unknown Albite35-kPoise Temp (° C.): 1793.2 1760.7 1749.9 1715.3 1723.8 1705.4Fictivation Temp (° C.): 1224.1 1209.5 1196.8 1206.8 1215.8 1205.7Refractive Index: 1.49821 1.49833 1.49955 1.50044 1.50084 1.50117 SOC(nm/MPa/cm): 31.99 31.55 31.44 31.49 31.25 32 CS at 50 μm 944.9 920.7944.7 1018.3 DOL/410° C./1 mm/ Annealed/Refined (MPa): Time to 50 μm DOLat 15.4 16.3 17.6 16.3 410° C./Annealed/Refined (h): PredictedIndentation 23 21 21 15 13 20 Threshold at 410° C./8 h/ Fictivated(kgf): Zircon Breakdown 1216 1224 1199 1205 1195 1220 Temperature (°C.): Zircon Breakdown 39 28 34 36 49 28 Viscosity (kPoise): Example 2526 27 28 29 30 Anneal Point (° C.): 628 629 630 635 629 640 Strain Point(° C.): 576 576 577 582 576 586 Softening Point (° C.): 894.5 898 896.4905.8 898.9 904.7 Density (g/cm³): 2.405 2.412 2.409 2.405 2.402 2.402CTE (×10⁻⁷/° C.): 76.4 74.4 76.5 75.9 75 75.3 Poisson's Ratio: 0.2250.219 0.206 0.207 0.223 0.214 Shear Modulus (Mpsi): 4.152 4.135 4.1654.099 4.212 4.126 Young's Modulus 10.173 10.079 10.048 9.893 10.29910.02 (Mpsi): 72-hr Pt Liquidus Temp 980 990 965 960 970 1000 (° C.):Liquidus Viscosity 970 970 950 945 950 995 (kPoise): Primary DevitPhase: 3223 2744 4960 5696 4346 2738 200-Poise Temp (° C.): UnknownUnknown Albite Albite Forsterite Unknown 35-kPoise Temp (° C.): 1725.21739.8 1729.2 1741.3 1744.1 1753.8 Fictivation Temp (° C.): 1213.21216.1 1215.4 1221.0 1221.4 1232.2 Refractive Index: 1.49840 1.499101.49860 SOC (nm/MPa/cm): 32.52 32.61 31.88 31.73 32.06 32.13 CS at 50 μm980.3 990.1 1006.9 1008.9 991.8 971.9 DOL/410° C./1 mm/ Annealed/Refined(MPa): Time to 50 μm DOL at 17.1 19.3 16.7 17.5 18.0 15.8 410°C./Annealed/Refined (h): Predicted Indentation 19 22 21 27 14 24Threshold at 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1220 12501215 1218 1231 1223 Temperature (° C.): Zircon Breakdown 32 21 35 37 3140 Viscosity (kPoise): Example 31 32 33 34 35 Anneal Point (° C.): 630632 623 634 628 Strain Point (° C.): 576 578 572 581 575 Softening Point(° C.): 903.8 909.1 888.9 909.4 912.8 Density (g/cm³): 2.402 2.4 2.4022.398 2.399 CTE (×10⁻⁷/° C.): 74.8 75.2 76 74.6 74.2 Poisson's Ratio:0.224 0.221 0.219 0.218 0.217 Shear Modulus (Mpsi): 4.121 4.118 4.1054.11 4.118 Young's Modulus 10.086 10.058 10.009 10.011 10.021 (Mpsi):72-hr Pt Liquidus Temp 990 1005 1000 1010 1005 (° C.): LiquidusViscosity 980 1000 990 1000 1000 (kPoise): Primary Devit Phase: 25692319 1680 1825 1896 200-Poise Temp (° C.): Unknown Unknown UnknownUnknown Unknown 35-kPoise Temp (° C.): 1769.7 1776.1 1756.4 1752.81754.2 Fictivation Temp (° C.): 1222.2 1231.5 1208.8 1226.4 1218.8Refractive Index: SOC (nm/MPa/cm): 32.08 32.11 32.08 32 32.21 CS at 50μm 970.3 962.8 947.1 940.8 953.0 DOL/410° C./1 mm/ Annealed/Refined(MPa): Time to 50 μm DOL at 16.3 16.3 17.8 17.1 17.0 410°C./Annealed/Refined (h): Predicted Indentation 22 23 25 24 24 Thresholdat 410° C./8 h/ Fictivated (kgf): Zircon Breakdown 1219 1232 1226 12361227 Temperature (° C.): Zircon Breakdown 37 35 27 31 31 Viscosity(kPoise):

The glasses described herein are resistant to both chipping andscratching, making it well suited for use in cover plates, touchscreens, watch crystals, solar concentrators, windows, screens,containers, and other applications that require strong and tough glasswith good scratch resistance.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

1. A glass comprising: a. at least about 50 mol % SiO₂; b. at leastabout 10 mol % R₂O, wherein R₂O comprises Na₂O; c. Al₂O₃, whereinAl₂O₃(mol %)<R₂O(mol %); and d. 3-4.5 mol % B₂O₃, wherein B₂O₃(mol%)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %, and wherein the glass is ionexchangeable.
 2. The glass of claim 1 wherein the glass has a zirconiabreakdown temperature that is equal to the temperature at which theglass has a viscosity in a range from about 30 kPoise to about 40kPoise.
 3. The glass of claim 1, wherein the glass comprises three-foldcoordinated boron cations.
 4. The glass of claim 1, wherein the glass ision exchanged and has a layer under a compressive stress of at leastabout 600 MPa, the layer extending from a surface of the glass into theglass to a depth of layer of at least about 30 μm.
 5. The glass of claim4, wherein the compressive stress is at least about 800 MPa.
 6. Theglass of claim 1, wherein the glass is ion exchanged and has a Vickerscrack initiation threshold of at least about 10 kgf.
 7. The glass ofclaim 1, wherein the glass is ion exchanged and has a Vickers crackinitiation threshold in a range from about 20 kgf to about 30 kgf. 8.The glass of claim 1, wherein the glass is ion exchanged and has aVickers crack initiation threshold in a range from about 30 kgf to about35 kgf.
 9. The glass of claim 1, wherein the glass comprises at leastabout 0.1 mol % of at least one of MgO and ZnO.
 10. The glass of claim1, wherein the glass comprises: from about 9 mol % to about 22 mol %Al₂O₃; from about 9 mol % to about 20 mol % Na₂O; from 0 mol % to about5 mol % K₂O; at least about 0.1 mol % MgO, ZnO, or combinations thereof,wherein 0≦MgO≦6 and 0≦ZnO≦6 mol %; and, optionally, at least one of CaO,BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.
 11. The glass ofclaim 1, wherein the glass contains less than about one inclusion perkilogram of glass, the inclusion having a diameter of at least 50 μm.12. A glass comprising: a. 66-74 mol % SiO₂; b. at least about 10 mol %R₂O, wherein R₂O comprises Na₂O; c. Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol%); and d. B₂O₃, wherein 4.5 mol %≧B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol%))≧3 mol %, and wherein the glass is ion exchangeable.
 13. The glass ofclaim 12, wherein the glass has a zirconia breakdown temperature that isequal to the temperature at which the glass has a viscosity in a rangefrom about 30 kPoise to about 40 kPoise.
 14. The glass of claim 12,wherein the glass is ion exchanged and has a layer under a compressivestress of at least about 600 MPa, the layer extending from a surface ofthe glass into the glass to a depth of layer of at least about 30 μm.15. The glass of claim 14, wherein the compressive stress is at leastabout 800 MPa.
 16. The glass of claim 12, wherein the glass is ionexchanged and has a Vickers crack initiation threshold of at least about10 kgf.
 17. The glass of claim 12, wherein the glass is ion exchangedand has a Vickers crack initiation threshold in a range from about 20kgf to about 30 kgf.
 18. The glass of claim 12, wherein the glass is ionexchanged and has a Vickers crack initiation threshold in a range fromabout 30 kgf to about 35 kgf.
 19. The glass of claim 12, wherein theglass comprises at least about 0.1 mol % of at least one of MgO and ZnO.20. The glass of claim 12, wherein the glass comprises: from about 9 mol% to about 22 mol % Al₂O₃; from about 3 mol % to about 10 mol % B₂O₃;from about 9 mol % to about 20 mol % Na₂O; from 0 mol % to about 5 mol %K₂O; at least about 0.1 mol % MgO, ZnO, or combinations thereof, wherein0≦MgO≦6 and 0≦ZnO≦6 mol %; and, optionally, at least one of CaO, BaO,and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %.
 21. The glass of claim 12,wherein the glass contains less than about one inclusion per kilogram ofglass, the inclusion having a diameter of at least 50 μm.
 22. A glasscomprising a. 66-74 mol % SiO₂; b. at least about 10 mol % R₂O, whereinR₂O comprises Na₂O; c. Al₂O₃, wherein Al₂O₃(mol %)<R₂O(mol %); and d.3-4.5 mol % B₂O₃, wherein B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≧3 mol %,wherein the glass comprises three-fold coordinated boron cations, andthe glass is ion exchangeable.
 23. The glass of claim 22, wherein theglass is ion exchanged and has a layer under a compressive stress of atleast about 600 MPa, the layer extending from a surface of the glassinto the glass to a depth of layer of at least about 30 μm.
 24. Theglass of claim 22, wherein the compressive stress is at least about 800MPa.
 25. The glass of claim 22, wherein the glass comprises at leastabout 0.1 mol % of at least one of MgO and ZnO.
 26. The glass of claim22, wherein B₂O₃(mol %)-(R₂O(mol %)-Al₂O₃(mol %))≦4.5 mol %.
 27. Theglass of claim 22, wherein the glass comprises: from about 9 mol % toabout 22 mol % Al₂O₃; from about 9 mol % to about 20 mol % Na₂O; from 0mol % to about 5 mol % K₂O; at least about 0.1 mol % MgO, ZnO, orcombinations thereof, wherein 0≦MgO≦6 and 0≦ZnO≦6; and, optionally, atleast one of CaO, BaO, and SrO, wherein 0 mol %≦CaO+SrO+BaO≦2 mol %. 28.The glass of claim 22, wherein the glass contains less than about oneinclusion per kilogram of glass, the inclusion having a diameter of atleast 50 μm.
 29. The glass of claim 22, wherein the glass is ionexchanged and has a Vickers crack initiation threshold in a range fromabout 20 kgf to about 30 kgf.
 30. The glass of claim 22, wherein theglass is ion exchanged and has a Vickers crack initiation threshold in arange from about 30 kgf to about 35 kgf.